1. Field of Invention
The present invention pertains to an assembly for facilitating microelectrode recordings in the brain during implantation of deep brain stimulation hardware.
2. Description of Prior Art
The use of chronic electrical stimulation in the treatment of a variety of neurological disorders, including, but not limited to, Parkinson's disease, dystonia, essential tremor, and chronic pain, has recently revolutionized the field of functional neurosurgery. New applications of this technology are being forecast by experts in the field at a rapid rate, and many applications have indeed already been envisioned. These new applications encompass many diverse neurological disorders, such as obsessive-compulsive disorder, obesity, anorexia, tinnitus, dyslexia, and many others. The efforts to control some of these illnesses are finally coming to fruition.
The surgery for afflictions most commonly being treated by functional neurosurgeons today, namely Parkinson's disease, essential tremor, and chronic pain, demands a high degree of spatial precision. These treatments require the targeting of different areas within the brain to produce positive outcomes. It has been recognized, and increasingly accepted, that chronic deep-brain stimulation holds several advantages over alternative methods of surgical treatment, namely lesioning, inasmuch as lesioning can only permanently destroy neuronal activity. In most instances, the preferred effect is to stimulate or reversibly block activity in nervous tissue. Electrical stimulation is reversible and adjustable, by modification of such electrical parameters as frequency, pulse width, and amplitude of the electrical current.
There are several methods currently available, both invasive and non-invasive, for targeting deep brain structures. The non-invasive methods generally rely on imaging technologies, such as computer tomography, magnetic resonance, or positron emission tomographic imaging. Each method has its advantages and disadvantages. The most ideal method would enable the visualization of the structure of interest at a high degree of spatial accuracy. The best commercially-available technique currently provides an accuracy greater than a millimeter. The invasive methods, which include micro- or semi-microelectrode recording, and macrostimulation, however, can optimally yield sub-millimeter precision. Specifically, microelectrode recording can be used to record individual cell characteristics at a spatial interval of a micron. Macroelectrode stimulation, on the other hand, activates or inhibits hundreds to thousands of cells at a time. As precision in locating the stimulation is most important component of success, most experienced medical centers performing these types of surgery utilize the technique of microelectrode recording (MER), in addition to other methods, to gain sub-millimeter spatial accuracy during the implantation of deep brain stimulating electrodes. Unfortunately, because of the time commitment and expense associated with using independent microelectrode recording (MER) systems the majority of neurosurgeons worldwide who are performing deep-brain stimulation procedures do not use the technique. They generally rely on only non-invasive neuroimaging (usually magnetic resonance imaging) as an initial targeting method, and then solely on macrostimulation to refine the final position of the permanent electrode. The potential lack of precision in locating the electrode is the most important predictor of a good surgical outcome, results may not be as optimal as those obtained by including MER in the targeting armamentarium.
In particular, the assembly and process for microelectrode recording comprises a passing a 1.1 mm microelectrode carrier tube to a predefined distance above the MRI-defined target within the brain, typically 15 mm above target, and performing MER via a microelectrode housed within the hollow center of the carrier tube. The typical microelectrode consists of a stainless steel wire, approximately 0.27 mm in diameter, with an insulating polyamide coating. Typically, it has an attached tungsten tip with a diameter of 1-3 .mu.m, and an impedance of 500 to 1000 kOhms at a typical frequency of 1000 Hz. This, in turn, is housed in a stainless steel carrier tube with an external insulating teflon coat and a total outer diameter of 1.1 mm. The microelectrode tip is used as one pole of an electrical circuit, and the distal exposed portion of the stainless steel carrier tube is used as the other pole to form a current loop. Cycling through the steps of microstimulation, recording, analyzing the results, and, if necessary, advancing of the lead to a new location is carried out until the specific ideal location is identified. While this provides the best localization of the deep brain stimulationlead, as suggested above, there are several technical reasons why many neurosurgeons do not routinely employ MER during the placement of deep-brain electrodes. First, recordings may take anywhere from 1 to 10 or more hours to perform. Second, the use of the microelectrode requires additional electronic equipment and the aid of a neurophysiologist. Third, there are some who believe that multiple microelectrode insertions and movements into and within thee brain may increase the complication rate of the surgery. While the tip of the microelectrode is only 1-3 microns in diameter, the carrier tube that houses the electrode is 1.1 mm in diameter, which is not an insignificant thickness considering that the size of a typical neuron is several magnitudes smaller. As such, most practicing functional neurosurgeons do not perform MER routinely believing that its possible risks outweigh its benefits.